Antimutator strains (i.e., strains that have lower mutation rates than the parental wild-type strain) are important tools in elucidating the precise mechanisms by which organisms produce mutations. In general, antimutators can be thought of as organisms that possess increased efficiency in certain cellular mutation-prevention systems. The well-defined genetics and molecular biology of the bacterium E. coli permit a systematic search for mutants that are thus affected. For example, by means of specific tests, mutants may be isolated that produce less errors of replication (by virtue of increased efficiency of DNA polymerase base selectivity, increased exonucleolytic proofreading, or increased DNA mismatch repair). Subsequent demonstration that these DNA replication mutants are antimutators for overall spontaneous mutagenesis would provide strong evidence that errors of DNA replication are a main source of spontaneous mutation. Analogously, mutants might be isolated possessing increased resistance against DNA damage from cellular metabolites, such as oxygen radicals. A demonstration that such mutants possess reduced overall mutation rates may be used to argue that oxidative damage is a main contributor to spontaneous mutation. The usefulness of such pathway-specific antimutators is not limited to spontaneous mutation but extends to the many forms of induced mutation. Using this approach, we have developed a specific assay to isolate mutants of E. coli that display reduced rates of DNA replication errors. These strains were found to carry a mutation in the dnaE gene, which encodes the DNA polymerase III that is primarily responsible for the (faithful) replication of the bacterial chromosome. It is our expectation that these strains will allow us to (i) define the specific contribution of DNA replication errors to spontaneous mutation, (ii) further define the mechanisms by which DNA polymerases can replicate DNA with high fidelity.